Memrise.com is one of the best web experiences I’ve ever had.
It’s a learning community where you can learn words and things from a vast spectrum of topics.

The idea is that when you meet a new word, a small seed is planted in your green house. As you spend more time with it, the seed sprout and grows (and is now housing in your short-time memory).

Memrise is full of clever things, and when you’re (according to their science) about to forget the memories, they send you a nice email with a direct link to the specific words or things, that you are about to forget.
Great service, love it!